1. Technical Field
This invention relates to public cordless telephone systems and more particularly to cordless telephone base units arranged for deployment in a public telephone system.
2. Description of the Prior Art
Telephone communications in public areas have traditionally been limited to a person using a public telephone where he or she could make either charge calls, pay calls or credit card calls. The mobility of the user has thus been limited to the immediate surroundings of the public telephone, as determined by the length of the telephone handset cord. Other alternatives, such as cellular telephone service and the new telepoint system, are expensive and therefore unattractive to many potential users.
One economical alternative to the public telephone is the new public cordless telephone system. Such a system is described in U.S. Pat. No. 4,953,198 which issued to J. J. Daly et al. on Aug. 28, 1990. The public cordless telephone system includes a plurality of public base units that are accessible by a plurality of portable or cordless telephone handset units. Each one of these handset units is capable of establishing communications with each one of the base units over a plurality of predetermined channels. Each one of these channels includes a radio frequency carrier signal that is modulated by switching signals and audio signals to and from a public base unit as appropriate. And the public base units are connected to telephone lines for providing conventional telephone communications for the handset units.
In the operation of the public cordless telephone system, communications between the handset unit and the public base unit is established by a user activating the handset unit which, in turn, interrogates the plurality of predetermined channels in order to establish communications with a public base unit on a nonbusy or available one of those channels. Once the handset unit selects an available channel, it transmits a service request which includes a handset identification code over this channel. A public base unit within the reception range of the handset unit receives the handset unit's identification code, appends its own identification code to this received code and then transmits the combined handset unit identification code and public base unit identification code back to the handset unit. The handset unit receives this combined code from the public base unit and then compares the handset portion of the received code with the code that it previously transmitted. If a favorable comparison of this code and the handset portion of the code received from the public base unit is obtained, communications is established between the handset unit and the public base unit on that selected channel.
If a handset unit happens to be within the reception range of two or more public base units that are capable or responding to the handset unit on the same channel, collision between these base units in responding to the handset unit may result. Thus if more than one public base responds to a handset unit at the same time, each code received by the handset unit is very likely to be rendered unintelligible by the other one or more received codes and the handset unit is forced to ignore all the receive codes and go through the process of generating another service request either on that channel or on another channel, hopefully with more favorable results the second time.
In order to prevent this collision between two or more public base units within the reception range of a handset unit requesting service, some public base units that are arranged in a cluster have been assigned to operate only upon one specific channel. Other public base units that are clustered and allowed to scan the available channels are controlled by a common control unit that prevents more than one of these public base units from attempting to respond to a service request coming from the same handset unit. Both of these arrangements have disadvantages, however. The demand for public cordless service is expected to grow over time and additional public base units for this service are expected to be periodically installed. Before these additional public base units may be installed close to other previously installed public base units, either exact knowledge of the assigned operating channels of the previously installed public base units or modification to the previously installed public base units will be required.
While the foregoing arrangements avoid contention between public base units in responding to a service request from a handset unit, it is desirable to have public base units capable of operation in close proximity to each other without interference and without the need for special configuration or modification of previously installed public base units as additional public base units are deployed in a public cordless telephone system.